Natural gas is obtained from underground reservoirs and pumped through pipelines to various industrial and commercial consumers. Much of the natural gas is utilized for heating purposes and, accordingly, requires a BTU content of only 900 to 1000 BTU per m.c.f. A natural gas stream composed mainly of methane and ethane is sufficient to achieve such heating values. However, much of the natural gas obtained from underground reservoirs contains substantial quantities of nitrogen and hydrocarbon components, such as ethane, propane, pentane and butane, which are heavier than methane. The heavier hydrocarbon components are industrially valuable in many processes, and accordingly, individual separation of them from the methane as a product is highly desirable. However, removal of the heavier hydrocarbons leaves a nitrogen and methane mixture that will not sell as such because of its low BTU content. Based on this problem, the nitrogen must be separated from the methane. The present invention provides a means of separating the nitrogen as a low BTU fuel stream and production of a methane product that meets the 900 to 1000 BTU requirements.
None of the prior art shows the process of the present invention for producing a nitrogen rich low BTU fuel, a methane product meeting the BTU requirements and recovery of the ethane-and-heavier hydrocarbons as product liquids.
It is an object of the present invention to teach a method of cryrogenic separation that lowers overall fuel consumption or horsepower to produce the cryogenic temperatures required.
It is a further object of the present invention to teach a method of cryogenic separation that permits broad latitude of operation. In particular, the ability to adjust the refrigerant composition to match the cooling and condensing characteristics of the feed in the process permits a degree of freedom not available in other process schemes more rigidly fixed or restrained by equipment designed for a specific process. This feature permits the process to process feed gases having a broad range of composition levels, without suffering in recovery efficiency.
It is yet another object of the present invention to teach a method of cryogenic separation where the turn-down capability is essentially unlimited.
It is yet a further object of the present invention to teach a method of cryogenic separation wherein the reduction in ethane-and-heavier hydrocarbon recovery efficiency is not nearly as pronounced on increasing through-put as it is with other systems.
It is yet another object of this invention to teach a method of separating nitrogen from methane and producing methane product having a BTU content of 900 to 1000 BTU and recovery of the ethane, propane and heavier hydrocarbons as a liquid product.